The polymerisation of olefins by bringing them into contact with a catalyst system in a reactor system comprising a gas phase reactor is known. In a fluidised bed gas phase polymerisation process, for example, a bed of polymer is maintained in a fluidised state by an ascending stream of fluidisation gas comprising the olefin, which gas exits the reactor and is then recycled.
The start-up of the polymerisation reaction in the gas phase is generally performed by introducing a pre-formed bed of polymer, known as a seedbed, to the reactor, fluidising this seedbed and forming a reaction gas mixture in the reactor, and then introducing a catalyst to initiate reaction.
Polymerisation may then be performed continuously; the catalyst system and make-up reactants and other reaction gas mixture components being introduced into the reactor continuously or discontinuously as required. The polymer produced may be withdrawn from the reactor either continuously or discontinuously. The pre-formed seedbed forms the initial fluidised bed but as reaction takes place and polymer solids are withdrawn this is replaced by a bed of the formed polymer.
The gas leaving the reactor is generally cooled before being recycled back to the reactor. In preferred processes it is cooled below a temperature at which liquid components condense out of the gas stream, and both the liquid and gaseous phases are recycled, wherein the vaporisation of the condensed liquid components in the reactor provides significant cooling to the reaction. The formation of condensed liquid from the gas exiting the reactor and the introduction of this condensed liquid into the reactor is generally known as “condensed mode” operation.
In the industrial production of polyolefins it is sometimes necessary to stop the polymerisation reaction. This may be for a number of reasons. For example, reactors may be shut-down for scheduled or unscheduled maintenance or cleaning of the reactor or other parts of the reaction system. Certain operations, such as some transition processes, also require the polymerisation to be stopped.
Impurities (“catalyst poisons”) such as water and/or oxygen may be introduced into the polymerisation reactor during these various operations. For example, if the reactor is opened during the shut-down impurities such as water and/or oxygen may enter the reactor. As another example, if a new seedbed is introduced during a transition it may contain water, oxygen or other impurities which have accumulated during storage of the seedbed. These impurities can inhibit the restarting of polymerisation, and can cause significant difficulties.
For this reason it is known to purge reactors and seedbeds to remove water and other impurities. It is also known to introduce compounds, known as “scavengers” which can react with the impurities and thereby remove them.
Purging methods include pressure purging with an inert gas such as nitrogen at an elevated temperature, or flow purging the reactor with nitrogen or other suitable inert gases at a reduced pressure. A vacuum can be placed on the reactor, preferably at elevated temperature, to reduce the level of water.
With respect to scavengers, alkyl aluminium and other alkyl metal compounds such as trimethyl aluminium, triethyl aluminium and diethyl zinc have been introduced to reactors to serve as scavengers for catalyst poisons before commencing polymerisation.
Hydrocarbons such as ethane, ethylene, propane, propylene, butane, isobutane, 1-butene, n-pentane, isopentane, n-hexane and 1-hexene can also be introduced and circulated in the gaseous state to increase the heat capacity of a circulating medium and thus speed drying. For example, WO2004007571 discloses a process for the polymerisation or copolymerisation in the gas phase of olefin(s) by bringing the said olefins(s) into contact, under polymerisation or copolymerisation conditions in a reactor in which the polymer or the copolymer is maintained in a fluidised bed and/or agitated with mechanical stirring, with a catalyst system, which process comprises a pre start-up operation characterized in that, prior to the introduction of the catalytic system in the reactor, the reactor is subjected to a cleaning treatment comprising the steps of introducing into the reactor an alkane having from 4 to 8 carbon atoms, circulating said alkane across the reactor under pressure and elevated temperature, depressurizing and purging the reactor.
Combinations of the above may be used. For example, the reactor may be purged to reduce water, and then a scavenger added to reduce the water still further.
An example of a process which includes purging and scavenging of an empty reactor (prior to introduction of a new seedbed) can be found in WO 00/58377.
An example of a process in which a seedbed is added to a reactor and then treated can be found in EP 180420. This document discloses a process for the start-up of polymerisation in the gas phase by bringing an olefin in contact under polymerisation conditions in a fluidised bed and/or with mechanical stirring with a charge powder (i.e. a seedbed) in the presence of a catalyst system of the Ziegler-Natta type. The charge powder used is dehydrated and subjected to a treatment by bringing the said charge powder into contact with an organoaluminium compound for a period of at least five minutes.